The invention relates to an interconnectable star coupler and to an optical communication network configured using such star couplers.
In recent years, local area networks (LAN) are gaining in popularly as a data communication network between computers and workstations installed relatively close to one another. "Ethernet.RTM." developed by Xerox Corporation is one such example.
FIG. 30 is a diagram showing an Ethernet-based network. In FIG. 30, reference numeral 20 designates a coaxial cable; 21, taps (branches); and 22, nodes (a terminal or station).
Each node 22 is connected to the coaxial cable 20 through the tap 21. To increase nodes 22, taps 21 must be additionally provided for connection to additional nodes.
Development in optical communications using optical fibers as transmission media is noticeable. Such excellent characteristics as wide bandwidth, low loss, high noise resistance of the optical fiber make the optical fiber suitable to LAN. However, since the optical fiber cannot provide taps as with the coaxial cable, it is difficult to configure a network similar to Ethernet.
To overcome this problem, a network in which separate terminals are provided for transmission and reception at each node and all the nodes are distributed by a star coupler has been proposed. See, e.g., G. Rawson, IEEE Transactions on Communications, Vol. COM-26, No. 7, July 1978, "Fibernet: Multimode Optical Fibers for Local Computer Networks."
FIG. 31 shows the proposed example of an optical communication network using a star coupler. In FIG. 31, reference numerals 23a, 23b designate optical fibers; 22, nodes; 24, a star coupler of a mixing rod type; and 25, terminals.
A signal from each node 22 is converted into an optical signal by each light-emitting element 22a and supplied to the star coupler 24 through each optical fiber 23a. These optical signals are mixed all together by the star coupler 24 and then distributed to the light-receiving elements 22b through the optical fibers 23b, respectively, converted to electric signals again, and supplied to the nodes 22, respectively, in the form of electric signals. Accordingly, the characteristic that a signal transmitted from a single node is transmitted to all the nodes (or multiple transmission) is provided, thus allowing a communication network similar to Ethernet to be implemented.
As shown in FIG. 32, various types of actual couplers implemented on, e.g., a glass substrate are reported. See, e.g., Wada, Okuda, and Yamasaki, 1980 National Convention of Institute of Electronics and Communication Engineers of Japan, p. 963 and E. Okuda, I. Tanaka, and T. Yamasaki, "Planar Gradiene-Index Glass Wave Guide and Its Applications to a 4-Port Branched Circuit and Star Coupler"; Applied Optics, Vol. 23, No. 11, pp. 1745-1748.
In FIG. 32, reference numeral 23 designates an optical fiber; 25, terminals; 26, a mixing section; 27, waveguides; and 28, a glass substrate. The waveguides 27 and the mixing section 28 are formed by diffusing thallium (T1) ions, etc., into the glass substrate 28.
Similarly, there is a star coupler that is formed by a different process from the star coupler formed on the glass substrate. This example is implemented on a plastic substrate such as polycarbonate and has the same function as the aforesaid star coupler. See, e.g., Takato and Kurokawa: "Technology for Fabricating Optical Waveguides Using High Polymers", O plus E, November 1984, pp. 78-83 and T. Kurokawa, N. Takato, S. Oikawa and T. Okada, "Fiber Optic Sheet Formation by Selective Photopolymerization"; Applied Optics, Vol. 17, No. 4, pp. 646-650 (1978).
However, an optical communication network using such a star coupler has the shortcoming that network expansion is limited by the number of terminals the star coupler has.
This is because interconnection between star couplers results in forming a closed loop within the transmission path and this causes phenomena such as oscillation and attenuating vibration.
For example, as shown in FIG. 33, if one star coupler A has three pairs of input/output terminals 5-6, 7-8, 9-10 and another coupler B has three pairs of input/output terminals 11-12, 13-14, 15-16, then the input terminals 10, 11 must be connected to the output terminals 12, 9 in a cross form to interconnect both star couplers. In FIG. 33, X.sub.k, x.sub.n designate input signals to the input terminals 10, 11; and Y.sub.k, y.sub.n, output signals from the output terminals 9, 12. The input signal X.sub.k supplied to the input terminal 10 of the star coupler A is outputted from the output terminal 9 as the output signal Y.sub.k while converted to a signal level determined by a transfer constant A.sub.kk from the input terminal 10 to the output terminal 9. The output signal Y.sub.k is applied to the input terminal 11 of the other star coupler B as the input signal x.sub.n. Similarly, in the star coupler B the input signal x.sub.n is outputted from the output terminal 12 as the output signal y.sub.n while converted to a signal level determined by a transfer constant B.sub.nn from the input terminal 11 to the output terminal 12. The output signal y.sub.n is applied to the input terminal 10 of the other star coupler A as the input signal x.sub.n. Thus, a closed loop shown in FIG. 34 is actually formed, circulating the signals and causing oscillation and the like.